Asteroid 2008 TC₃, not a polymict ureilitic but a polymict C1 chondrite parent body? Survey of 249 Almahata Sitta fragments

Abstract

On October 7, 2008, the asteroid 2008 TC 3 exploded as it entered the Earth's atmosphere, producing significant dust (in the atmosphere) and delivering thousands of stones in a strewn field in Sudan, collectively known as the Almahata Sitta (AhS) stones. About 600 fragments were officially recovered in 2008 and 2009. Further rocks were collected since the fall event by local people. From these stones, 249 were classified at the Institut f ür Planetologie in M ünster (MS) known as MS-xxx or MS-MU-xxx AhS subsamples. Most of these rocks are ureilitic in origin (168; 67%): 87 coarse-grained ureilites, 60 fine-grained ureilites, 15 ureilites with variable texture/mineralogy, four trachyandesites, and two polymict breccias. We identified 81 non-ureilitic fragments, corresponding to 33% of the recovered samples studied in M ünster. These included chondrites, namely 65 enstatite chondrites (43 EL; 22 EH), 11 ordinary chondrites (OC), one carbonaceous chondrite, and one unique R-like chondrite. Furthermore, three samples represent a unique type of enstatite achondrite. Since all AhS stones must be regarded as individual specimens independent from each other, the number of fresh ureilite and enstatite chondrite falls in our meteorite collections has been increased by several hundred percent. Overall, the samples weigh between <1 and 250 g and have a mean mass of ~15 g. If we consider-almost 15 years after the fall-the mass calculations, observations during and after the asteroid entered the atmosphere, the mineralogy of the C1 stones AhS 91A and AhS 671, and the experimental work on fitting the asteroid spectrum (e.g., Goodrich et al., 2019;Jenniskens et al., 2010;Shaddad et al., 2010), the main portion of the meteoroid was likely made of the fine-grained (carbonaceous) dust and was mostly lost in the atmosphere. In particular, the fact that C1 materials were found has important implications for interpreting asteroid 2008 TC 3 's early spectroscopic results. Goodrich et al. (2019) correctly suggested that if scientists had not recovered the "water-free" samples (e.g., ureilites, enstatites, and OC) from the AhS strewn field, 2008 TC 3 would have been assumed to be a carbonaceous chondrite meteoroid. Considering that the dominating mass of the exploding meteoroid consisted of carbonaceous materials, asteroid 2008 TC 3 cannot be classified as a polymict ureilite; consequently, we state that the asteroid was a polymict carbonaceous chondrite breccia, specifically a polymict C1 object that may have formed by late accretion at least 50-100 Ma after calcium-aluminum-rich inclusions.

40Ar/39Arages ofL4,H5,EL6,and feldspathic ureilitic clasts from the Almahata Sitta polymict ureilite (asteroid 2008TC3)

“…The clast ages are mute as to where the heating of chondritic clasts ended and argon closure was reached, that is, on the parent bodies or on AhS progenitor 2008 TC 3 , and under what circumstances, that is, in a slowly cooling parent body or in collisional fragment rapidly cooling from higher temperatures. We favor the former choices, which are consistent with the conclusions of Goodrich, Sanborn, et al (2021) and Goodrich, Kring, et al (2021) and Bischoff et al (2022) that AhS acquired its xenoliths 50-60 Ma after the formation of UDBs.…”

“…Horstmann and Bischoff (2014) do not address what appears to be an implied delay between UPB disruption and the formation of 2008 TC 3 . In more recent work, Bischoff et al (2022) specify this time interval, stating that 2008 TC 3 was "a polymict C1 object that may have formed by late accretion at least 50-100 Ma after calcium-aluminum-rich inclusions." Goodrich, Sanborn, et al (2021) and Goodrich, Kring, et al (2021) argue that UDBs, including the progenitor(s) of both typical polymict ureilites and 2008 TC 3 , formed ~5 Ma after CAI.…”

“…However, Goodrich et al (2019) showed that, within error, it is also consistent with 2008 TC 3 being dominated by C1 carbonaceous chondrite material similar to some AhS clasts. Based on the latter observation, Bischoff et al (2022) argued that 2008 TC 3 should be classified as a polymict carbonaceous chondrite breccia and therefore its genesis must be distinct from that of polymict ureilites. In contrast, Goodrich, Bottke, et al (2021) argued that even if dominated by C1 material, 2008 TC 3 could have originated in the same regolith environment as typical polymict ureilites.…”

The chondrite breccia of Antonin (L4‐5)—A new meteorite fall from Poland with a heterogeneous distribution of metal

“…However, as mentioned before, the brecciation is difficult to recognize, and Antonin does not belong to the meteorites with complex brecciation textures. The parent body was certainly less brecciated than asteroid 2008 TC 3 (which is the parent meteoroid of the Almahata Sitta stones) or the parent body of Kaidun, which both include a great number of clasts or stones of different meteorite classes and types (e.g., Bischoff et al, 2010;Bischoff, Bannemann, et al, 2022;Goodrich et al, 2014;Horstmann et al, 2010;Horstmann & Bischoff, 2014;Zolensky et al, 1996Zolensky et al, , 2010Zolensky & Ivanov, 2003). It was also less brecciated than the parent bodies of other complex breccias among the ordinary chondrites, which also contain different rock types (e.g., Bischoff et al, 1993Bischoff et al, , 2006Bischoff et al, , 2018Bischoff, Dyl, et al, 2013;Metzler et al, 2010Metzler et al, , 2011Rubin et al, 2005Rubin et al, , 2017.…”

Chondrite diversity revealed by chromium, calcium and magnesium isotopes

“…We also determined their Ca and Mg isotope compositions to test this hypothesis. The sample suite analyzed in this study includes: 1) one EC, Sahara [SAH] (Patzer et al, 2001), and two enstatiterich stones (MS-MU-019 and MS-MU-036) (Bischoff et al, 2022;Harries and Bischoff, 2020) from the Almahata Sitta strewn field (Harries and Bischoff, 2020;Horstmann and Bischoff, 2014). Note that BUC 10933, MIL 07513 and MIL 090001 are currently classified as CR2 chondrites in the Meteoritical Bulletin, but their classifications are still under debate (Alexander et al, 2013).…”